Thanks for trying that out. On your custom parent entity for example p1 did you add the field you want to display in the dropdown to the default lookup field group like this? Did you add a text type field or some other type?
Can you refresh the datasource in PowerApps web player/studio or try a new AFD. This is what I see on the lookup field/relationship for my child entity c1. I am assuming you are trying this in your new environment not the legacy environment.
Another issue is: even when you have PrimaryID as Value1 - dont change anything and just change the form, you get an error that value is not set. You have to, every time you open the form, open dropdownlist, reselect value, and save. Only then it works.